It has been the practice to treat brittle material in accordance with techniques known from FIG. 6 of EP-A-84 383. The material crushed in the roller mill is delivered directly to a screen classifier. The oversize fraction is led from the screen classifier to the tube mill (a proportion can also be returned to the roller mill). The proportion of the product passing through the screen classifier passes together with the material discharged from the tube mill to a sifter, and the fines from the sifter form the finished material, whilst the tailings are returned to the tube mill (or a proportion thereof to the roller mill).
A method of this type has certain disadvantages. Practical experiments show that the agglomerates forming the oversize fraction on the screen classifier contain a considerable proportion of fines which is not separated off in the screen classifier in view of the stability of the agglomerates. Therefore the material reaching the tube mill is poorly calibrated. Because of the high tailings recycle factor it is necessary for the roller mill to be of large dimensions.
Operational experience with roller mills which have ball mills arranged after them also shows that ball mills react comparatively sensitively to fluctuations in the coarse grain content. This applies equally for ball mills operating by the dry process and by the wet process.
A further disadvantage of this known method is that in view of the size of the agglomerates the oversize fraction precipitated in the screen classifier can only be conveyed to the tube mill pneumatically or hydraulically at very high energy costs. This disadvantage is significant above all when for reasons of plant layout the roller mill and the tube mill have to be arranged distant from one another (for example because in a plant for the manufacture of cement the roller mill has to be arranged in the proximity of the clinker cooler and thus spatially far removed from the closed-circuit grinding plant containing the tube mill or because in a mine the roller mill has to be installed at the working face but the tube mill is located in the dressing plant--equally far away from the roller mill).